The Hydrolysis of Ferric Complexes. Magnetic and Spectrophotometric Studies of Aqueous Solutions of Ferric Salts

Ferric chloro, ferric aquo, and ferric sulfato complexes in acidic solutions have been hydrolyzed at temperatures below 70°C to form polynuclear complexes of iron(III). Magnetic and spectrophotometric studies have then been carried out in order to clarify the nature of the polynuclear complexes. From these studies, their empirical formulas are presumed to be Fe2(OH)2Cl2O, Fe2(OH)3(NO3)3, Fe2(OH)3(SO4)3⁄2, and Fe3(OH)2(SO4)7⁄2. The iron ions in these polynuclear complexes are in a high spin state, as are those in the ferric complexes, iron oxides, and oxyhydrates. It is proposed that, in all these polynuclear complexes except Fe3(OH)2(SO4)7⁄2, edge-shared octahedral dimer units are antiferromagnetically linked with one another by hydroxy or oxo bridging. When the hydrolysis temperature is raised, these polynuclear complexes are further hydrolyzed to form precipitates. The complexes of Fe2(OH)2Cl2O, Fe2(OH)3(SO4)3⁄2, and Fe3(OH)2(SO4)7⁄2 give β-FeOOH, α-FeOOH, and RFe3(OH)6(SO4)2 (R=Na, K or NH4) respectively, whereas the complex of Fe2(OH)3(NO3)3 gives α-FeOOH or α-Fe2O3, depending on the temperature.